WALKER — They look prehistoric as they feed just five miles from downtown Grand Rapids — in pastures, near swamps, in apple orchards, just outside a major county park, next to suburban homes.

Since the 1930s, these rusty but tireless oil wells have drawn sweet crude -- about 20 million barrels -- from limestone 1,900 feet under the Walker Oil Field, stretching from Kent County's Millennium Park northwest into Ottawa County.

Now, with crude oil selling for more than $130 a barrel, the rush is back on. Michigan prices are lower -- hovering around $90 -- but are on a steady rise.

And West Michigan's oil "barons" are working their wells harder than ever to tap what is left down there -- focusing not only on the Walker Oil Field, but at fields in northern Allegan County.

"It's American crude, anyway," said Ken Langerak, owner of Great Lakes Petroleum, which operates several dozen small wells. "Every barrel we pump out of the ground is one less barrel you have to buy from Saudi Arabia."

Some operators are using vacuum pumps to go after more crude; some have injected bacteria into wells, hoping to break down thick oil and make it easier to bring to the surface; some smaller operators can afford to use acid to eat away oil-bearing limestone.

"These days, everybody's trying to get the most that they can out of their wells," said professor William Harrison, director of the Michigan Basin Core Research Laboratory at Western Michigan University.